Motor driven tapping machines have been utilized for cutting circular holes in pipe sidewalls from at least 1971 when U.S. Pat. No. 3,614,252 issued. This patent discloses a tapping machine that has a feed screw for moving a boring bar axially upon rotation of the boring bar relative to the feed screw. A drive sleeve rotates the boring bar to provide such relative rotation and to rotate a cutter attached to the boring bar to cause the cutter to cut a hole in a pipe. The boring bar may have either a fixed or a variable feed rate depending upon the design of the tapping machine.
The tapping apparatus disclosed in U.S. Pat. No. 3,614,252 does not provide for pressure equalization and therefore when penetration is made into a pipeline that is under pressure, the internal pressure of the pipeline is applied to the boring bar and this pressure must be overcome by the motor used to advance the boring bar.
Tapping machines for use underwater have been known since at least 1986 when U.S. Pat. No. 4,579,484 issued, the patent being entitled "Underwater Tapping Machine". This patent discloses a tapping machine body equipped for attachment to a fitting secured to a pipeline and has a rotatable boring bar within the tapping machine body. A gear case is affixed to the body to activate the boring bar. A containment vessel is supported by the tapping machine body and has a first opening that communicates with the interior of the tapping machine and a second opening that communicates with the environment, that is, with a body of water, such as a lake, river or ocean, in which the tapping operation is carried out. A fluid barrier is provided in the containment vessel separating the first and second openings.
The portion of the containment vessel that is in communication with the interior of the boring bar apparatus is filled with a lubricant consistent with the lubricant normally employed in the boring tool so that ambient pressure experienced by the boring tool is equalized with the interior of the boring tool. In U.S. Pat. No. 4,579,484 the equalization pressure from the containment vessel is applied both to the interior of the boring bar body and at the same time to the interior of the gear case. The disclosures and teachings of U.S. Pat. No. 4,579,484 have been important in the advancement of the use of tapping machines in underwater applications.
The problem encountered when cutting an opening in a wall of a pipeline that is under pressure (referred to in the industry as shot "tapping") was addressed in U.S. Pat. No. 5,439,331 that issued on Aug. 8, 1995, entitled "High Pressure Tapping Apparatus". This patent discloses and teaches an apparatus for tapping an opening into an existing pipeline that may be under high internal pressure and provides a tapping machine body having a feed screw and a boring bar therein with means to rotate the feed screw and the boring bar to tap the pipeline, including means for sensing the pipeline pressure and for using the sensed pressure to balance the force applied against the boring bar resulting from the internal pressure of the pipeline.
The above referenced patents provide good background information as to the state of the art to which the present invention pertains.